Description of AI Behavior

Sim characters are semi-autonomous and attempt to lead normal lives involving eating, sleeping, bathing, going to the bathroom, working, having fun, and participating in relationships. Their behavior is sub-optimal (they randomly choose interesting things to do), purposely requiring the player to intervene as part of the game. Sims are highly influenced by their social network and their environment.

Notable Behaviors

The Sims ability to autonomously participate in daily activities regardless of the arrangement of the environment is impressive. In particular, Sim characters can intelligently interact with any object in the game since the interaction logic is stored within each object rather than inside the Sim (Smart Object / Smart Terrain architecture). Moreover, each Sim has a somewhat complex architecture involving personalities and needs that help drive the Sim through everyday life.

Simlish Language

The Sims speak a fictional language called Simlish which was developed by Maxis. The language takes inspiration from Ukrainian, French, and Tagalog. Simlish was created to avoid recognizable repetition in phrases and to avoid the cost of localizing the dialog for other regions. What is remarkable about Simlish is how the player can appreciate the tone and get the gist of what the Sims are saying. This was combined with "speech bubbles" containing various topical icons. Sometimes conversing Sims would seem to "reinforce" a topic the one brought up. Other times, a Sim would seem to "negate" a topic by having a red slash through the icon and playing a negative-sounding Simlish clip -- perhaps along with a negative-looking animation. The result was the appearance of coherent, animated conversation.

Architectures

Agent Architecture

Sims have four key aspects to their personal agent architecture: Personality, Needs, Skills, and Relationships.

Personality

Each Sim has individual ratings across five personality traits. The traits are: Sloppy to Neat, Shy to Outgoing, Serious to Playful, Lazy to Active, and Mean to Nice.

Personality traits affect what Sims choose to do autonomously and how much benefit a Sim receives from a given interaction. For example, a more serious Sim will derive more "Fun" from playing chess than playing a pinball machine.

Needs

Happiness Weights in The Sims

Sims have two classes of Needs: Physical and Mental. Physical needs include Hunger, Comfort, Hygiene, and Bladder. Mental needs include Energy, Fun, Social, and Room.

Each Need is rated on a scale of -100 to 100 based on the history of the Sim. For example, if the Sim is fully rested, his Energy is at 100. All eight needs are weighted through individual response curves and summed together to generate an overall Mood (on a scale from -100 to 100). For example, the response curve weight for Bladder is near zero when Bladder is from 0 to 100, resulting in almost no effect on Mood when the Sim's Bladder is positive (when the Sim doesn't have to urinate). However, the Bladder response curve below zero increases exponentially as it approaches -100 (when the Sim must urgently urinate). Thus negative values for Bladder increasingly contribute to a negative mood but positive Bladder values have virtually no effect on mood.

Physical Needs

Hunger can be satisfied by eating. A Sim will die if Hunger drops to the lowest point.

Bladder decreases steadily during the day and can be satisfied by urinating.

Mental Needs

Energy decreases while awake and can be increased by sleeping.

Fun can be increased by many objects, such as playing chess or pinball.

Social can be increased by interacting with other Sims (if the interaction is positive).

Room reflects the design and content of the Sim's current surroundings.

Skills

Sims can acquire various degrees of Skills which affect their interactions. Skills include Cooking, Mechanical, Charisma, Body, Logic, Creativity, and Cleaning. For example, if a Sim has a low Cooking skill, they will catch the stove on fire when they try to cook.

Relationships

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Smart Objects and Smart Terrain

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The Sims introduced an architecture known as "smart objects" to the game world. The main thrust was to offload a number of functions from the agent onto the objects in the environment. Each inanimate object in the game (that the Sim can interact with) contains two important categories of information: what benefit the object can provide for the Sim and how the Sim interacts with the object.

Each object, in effect, broadcasts its benefit to nearby Sims. For example, the toilet broadcasts that it can increase Mood by 26, if urinated in will increase Bladder by 40, if cleaned will increase the Room score by 30, and if unclogged will increase the Room score by 40. Further, the toilet contains the animations and step-by-step instructions on how the Sim can use the Toilet for each action (urinate, clean, or unclog). This architecture allows Sims to intelligently use any combination or variations of objects and also allows for expansion packs of unanticipated objects without updating the main game code.

Happyscape

1. "Happyscape" in The Sims

The "terrain" created by various objects broadcasting their services is called the "Happyscape" by Will Wright and is loosely referred to as Smart Terrain. Sim characters can increase their happiness (Mood) by hillclimbing toward local maxima created by what particular objects have to offer. Once the Sim uses the object (which could also be another Sim), the current hill normalizes and they are off to climb another local maxima.

The three images here demonstrate a Sim's Happyscape over time as objects are used. The words below the center male Sim show his current Needs (with larger text representing a more urgent Need).

2. "Happyscape" in The Sims

In image 1, the Sim has three Needs (Hunger, Social, and Fun which are labeled as "Hungry", "Lonely", and "Bored", respectively). His most urgent Need is Hunger. Consequently, the refrigerator appears to be the most attractive object in the landscape. The Sim is pulled to the refrigerator and uses it to satisfy his hunger. Once he does, the refrigerator is no longer attractive to him because his hunger has been dramatically reduced, as shown in the next image.

3. "Happyscape" in The Sims

In image 2, the Sim's most urgent Need is Social (labeled as "Lonely" in the image). Consequently, the female Sim is the most attractive object in the landscape and the Sim is pulled to the female Sim. Once the Sim has satisfied his Social Need, the female Sim's attractiveness becomes reduced, as shown in the next image.

In the final image 3, the Sim's most urgent Need is Fun (labeled as "Bored"). The landscape has two objects that can satisfy this Need: the pinball machine and the chess board. Because the Personality of the Sim is more Playful than Serious, the pinball machine is more attractive than the chess board (and will result in better satisfying the Sim's Fun Need).

This example is intentionally simple in order to demonstrate the concept. In reality, there are often dozens of objects in the proximity of the Sim, each of which is broadcasting one or more "signals" about a Need that it can satisfy.

Pulled from Task to Task

Some objects, such as food, need to be processed before they can be directly used by the Sim. In this case, the food contains instructions on how it needs to be prepared (for example, cooked) before the Sim can eat it. In fact, eating is a complex process that pulls the Sim from one activity to another in the following manner. First, the Sim is hungry and the refrigerator broadcasts that it contains food. The Sim uses the refrigerator and gets the food. The food tells that Sim that it must be cooked and the stove broadcasts that it can cook food. After using the stove, the cooked food directs the Sim to place the food on a table surface and the Sim must sit down to eat it. Once the Sim is done, he will dispose of the leftover food if they happen to have a Neat personality (otherwise the dirty dish will sit on the table, flies will come, and the Room score will be decreased).

Affordance

The smart object architecture has parallels to the concept of affordance popularized by Donald Norman in his book The Design of Everyday Things. An example of an affordance in real life is the handle on a teapot which telegraphs to the human how to interact with it. The teapot affords a particular way of holding it and tipping it, thus conveying to the human how to use it. Similarly, a handle on a door that can be grasped is telling the human to grab the handle and pull (which sometimes is incorrect for poorly designed doors). Conversely, a flat panel on a door that can't be grasped is telling the human to push on the door. In The Sims, smart objects can be viewed as performing a similar duty in which they tell a Sim how the object should be used.

Post-Mortem

What Worked

The semi-autonomous Sims have just enough AI to keep them doing interesting things, but are sub-optimal enough that the player has fun directing them. This balance proved to be a winning formula that keeps the player involved, yet doesn't require constant micromanaging.

Smart Objects

Smart objects were a stroke of genius that helped continue the Sims franchise well beyond the initial boxed titles. Authors, both from Maxis and from the general public, could create a new object and tag it with affordances and the relevant animations, and drop it into the game. The Sims would immediately know what to do with it. This allowed for seven expansion packs and countless items that could be downloaded from online.

What Didn't Work

One complaint about the game was that children never grew up to become adults and adults never aged. Some people found this surprising and disappointing, however it may have provided a better play experience for the average player. This was addressed in later titles in the franchise, notably in The Sims 3.

Another complaint was that there is no concept of weekends, with Sims having to go to work every day (however they could miss one day at a time without being fired.

Pathfinding

There were some pathfinding issues, such as Sims getting stuck on chairs that were in the path.

Reception by Public

People were drawn to both the control that one could exhibit over the Sims and the promise of being able to sit back and observe the autonomous behavior. Groundbreaking as it was, the public was fascinated that the people in the game were "living" their lives and making intelligent decisions. For the most part, game characters to this point had not had the need to exhibit any self-awareness much less try to satisfy desires. This led many people to initially view this game as a fishbowl -- that is, "set them up and see what happens."

On the other hand, at times the autonomous behaviors were less than adequate and the player would feel the need to "bail them out". When this happened, it was often obvious that the AI was not making the correct decisions. This occasionally would burst the bubble of "little living people". However, most people were quite content to treat The Sims as a sort of animated dollhouse.

Another complaint, related to the above observation, was that there often wasn't enough time in the day for the Sims to perform anything other than mundane, life-maintenance tasks. While this is more a design decision than an AI-related one, it was somewhat enabled by the occasional failure of the AI to perform optimally.

Despite the minor annoyances surrounding what, at the time, was groundbreaking AI, The Sims became the best selling PC game in history when it came out, selling more than 6.3 million copies.